Editor's Note
The impact of low temperatures on lithium-ion power batteries and proton exchange membrane fuel cell (PEMFC) stacks is quite different. In general, low temperatures have a more significant negative effect on the performance of lithium-ion batteries compared to PEMFCs.
2.4 Comparison of Low-Temperature Performance Between Lithium Batteries and Fuel Cells
Due to their widespread use in vehicles, low-temperature performance is a critical technical parameter for power batteries. The low-temperature behavior of lithium-ion batteries is primarily influenced by temperature effects on the conductivity, ion diffusion coefficient, and electrolyte conductivity of the electrode materials. At lower temperatures, the electrolyte becomes more viscous, reducing its ionic conductivity and increasing cell polarization. As a result, the performance of lithium-ion batteries drops significantly near 0°C, and they typically cannot operate effectively at -20°C. Frequent charging and discharging under such conditions can severely degrade battery life and may lead to lithium plating on the anode, posing serious safety risks. Improving low-temperature performance often comes at the cost of other key parameters like cycle life and energy density, which can increase production costs.
In contrast, the issue with PEMFCs at low temperatures is known as "cold start." This refers to the ability of a fuel cell electric vehicle (FC-EV) to restart within a certain time after being shut down. Cold start remains a major technical challenge due to ice formation in the stack, which hinders electrochemical reactions. However, once the PEMFC is started, the heat generated during operation quickly raises the stack temperature to the normal range of 80–90°C, even in extremely cold environments. This makes PEMFCs more resilient to low temperatures than lithium-ion batteries in terms of operational stability.
Extensive research has been conducted on cold start performance below the freezing point of PEMFCs. Currently, Daimler-Benz has demonstrated successful cold starts at -25°C, while Toyota, Nissan, and Honda have achieved starts at -30°C. The target for ordinary vehicles is to achieve cold starts at -40°C, indicating that further improvements are still needed for FC-EVs.
From the above analysis, it is clear that low temperatures affect lithium-ion batteries and PEMFC stacks in fundamentally different ways. Lithium-ion batteries are much more sensitive to cold, while PEMFCs can recover quickly once started.
2.5 Reliability Comparison Between Lithium-Ion Batteries and Fuel Cells
Battery reliability refers to the probability that a battery will lose its ability to store electrical energy. While safety is closely related to reliability, they are not the same concept. A safety incident in a lithium-ion battery will inevitably lead to a loss of energy storage capacity. However, the degradation of energy storage capacity is not always caused by safety issues—such as capacity "dipping" due to internal failures.
Power battery systems consist of hundreds of individual cells connected in series and parallel. This configuration greatly amplifies the system’s reliability concerns. Based on limited usage data from domestic electric vehicles, the reliability of large-scale battery systems remains unsatisfactory. The root cause of these reliability issues is linked to the safety factors discussed earlier, which are determined by the inherent characteristics of the battery chemistry.
Before discussing the reliability of PEMFCs, it's worth noting that fuel cells have a long history of reliable performance. PEMFC technology was developed based on alkaline fuel cell (AFC) technology, which was originally designed for aerospace applications. In the 1970s, United Technologies Corporation (UTC) successfully applied AFC stacks to the U.S. space shuttle, and later, third-generation AFC systems became the standard power source for the shuttle, demonstrating the high reliability of fuel cell technology.
Another area where batteries are used extensively is in conventional submarines. High-energy battery systems allow submarines to remain submerged longer, providing significant tactical advantages. Most modern conventional submarines use lead-acid batteries, while no country currently uses lithium-ion batteries as main power sources in conventional or nuclear submarines. The author believes this is due to safety and reliability concerns rather than cost.
For example, Japan's "Canglong"-class submarine initially planned to use lithium-ion batteries but later switched to PEMFC due to technical and budgetary challenges. Similarly, many countries are now adopting PEMFC as the primary power source for AIP (Air Independent Propulsion) systems in their submarines.
Most AIP submarines currently in service or soon to be deployed use PEMFC as their main power system. Germany's 212 and 214-class submarines, for instance, use PEMFC stacks developed by Siemens and HDW. These systems have evolved over decades to become highly reliable, meeting the strict requirements of military applications.
Looking at the development of AFC and PEMFC, fuel cells were originally designed as large-scale power sources, evolving from small devices like mobile phone batteries into powerful energy systems. This path is fundamentally different from that of secondary batteries, which are typically used in smaller, portable applications.
Lithium-ion batteries, on the other hand, have lower energy density when scaled up, due to the limitations of their electrochemical systems. In contrast, PEMFCs benefit from advanced auxiliary systems that enhance mass transfer, humidification, drainage, and temperature control, leading to better power density in larger systems.
Given these differences, it is reasonable to conclude that lithium-ion batteries are best suited for medium and small power storage applications, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and small electric vehicles. Meanwhile, PEMFCs, which were developed as large-scale power sources from the beginning, are more suitable for high-power applications like transportation and military systems.
In summary, while both lithium-ion batteries and fuel cells play important roles in modern energy systems, their design philosophies and application areas are distinct. Understanding these differences helps guide the appropriate use of each technology in various sectors.
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